1. Field of the Invention
The present invention relates to sol-gel precursors for the production of lead-based perovskites such as lead zirconate titanate (PZT) as well as methods for making lead-based perovskite films.
2. Brief Description of the Prior Art
In recent years, piezoelectric micro-electro-mechanical systems (MEMS) have seen rapid growth in micro-actuators and sensor applications. For MEMS, a piezoelectric thin film must be deposited on a silicon substrate and integrated in the silicon-based micro-fabrication process. The current material of choice for the piezoelectric thin film is lead zirconate titanate (PZT) because of its high dielectric constant and piezoelectric coefficients. Although the dielectric constant of bulk commercial PZT can be over 3000 and its d33 piezoelectric coefficient over 600 pm/V, about two orders of magnitude higher than those of commonly used quartz and ZnO, most PZT films only exhibit a dielectric constant of 800-1500, and a piezoelectric coefficient, d33=190-250 pm/V, 20 to 40% of the d33 of the bulk commercial PZT. This is primarily due to thin-film related problems such as the interfacial diffusion/reaction, substrate pinning, and lack of control over film stoichiometry.
Compared with vacuum-based vapor deposition methods such as pulsed laser ablation, electron-beam evaporation, ion-beam deposition, radio-frequency (RF) planar magnetron sputtering, and metallorganic chemical vapor deposition (MOCVD), the solution-based sol-gel method is advantageous in terms of simplicity, economy, and the ease with which the composition can be controlled. The main difficulty of the sol-gel method, however, is lead loss during the repeated heat treatments required by the method.
In addition, cracking of the coating after heat treatment may limit the film thickness that can be achieved using sol-gel methods. Although viscous additives such as polyvinylpyrrolidone (PVP) can increase the film thickness they tend to degrade the film properties.
In order to make thick lead-based ferroelectric/piezoelectric films without the need for viscous additives, a precursor solution that has the right viscosity for film rigidity during sintering is required. In addition, both lead loss and interfacial reactions can change the film stoichiometry and degrade the film properties.
U.S. Pat. No. 5,536,963 discloses the preparation of a perovskite lead zirconium titanate (“PZT”) piezoelectric layer by sol-gel deposition techniques involving sequential deposition and curing of a plurality of PZT layers.
U.S. Pat. No. 6,523,943 also discloses the preparation of a perovskite PZT piezoelectric layer by sol-gel deposition techniques involving sequential deposition and curing of a plurality of PZT layers. Suitable solvents are combinations of polar and non-polar solvents. Alcohols such as ethylene glycol and amides are mentioned as potentially useful polar solvents Non-polar solvents include toluene, hexane, cyclohexane and acetonitrile.
U.S. Pat. No. 5,585,136 also discloses the preparation of a perovskite PZT piezoelectric layer by sol-gel deposition techniques involving sequential deposition and curing of a plurality of PZT layers. A mixture of polyethylene glycol and glycerol was employed as the solvent in Example 1 to provide a PZT film.
U.S. Pat. No. 4,756,147 discloses the preparation of a perovskite PZT layer using a sol-gel process wherein lead acetate is dissolved in dehydrated acetic acid, zirconium piopoxide and titanium isopropoxide are added and a mixture of ethylene glycol and propanol was added to form a stable sol. Additional ethylene glycol is added to adjust the viscosity of the solution.
Therefore, to obtain superior dielectric, ferroelectric, and piezoelectric properties the present invention controls the viscosity of the precursor solution and/or the composition of the precursor to provide the desired film stoichiometry after multiple heat treatments.
One object of certain embodiments of the invention is to develop a new sol-gel precursor for lead-based perovskite thin films on a silicon-based substrate with superior dielectric and piezoelectric properties for various dielectric, ferroelectric, and electromechanical systems (MEMS), as well as for micro-sensor applications.
One solution offered by certain embodiments of the invention is a sol-gel process that allows deposition of thick lead-based ferroelectric/piezoelectric films with superior dielectric, ferroelectric properties and without the need for viscous additives which may adversely affect film properties or performance.